The invention is concerned with a novel optical filament for use in optical communication systems.
Structurally, an optical filament and an optical fiber are similar in that each is a composite of a light transmitting core, usually glass, and a compatible cladding of lower refractive index material. The refractive index differential causes light rays entering the core to be repeatedly refracted at the core-cladding interface, and thus travel the length of the core. However, the light attenuation level in an optical fiber is sufficiently high, due to impurities and imperfections in the glass, that an optical signal can be transmitted only a short distance. Even then the clarity is usually inadequate for optical communication.
Recent technical advances in glass forming and processing techniques have evolved glasses of such a low impurity level that optical signals can be transmitted over long distances without substantial loss or distortion. Optical filaments thus produced are commonly known as optical waveguides and hold great promise in the communications field. Decreasing production costs are opening broad areas of application for this new communications tool. As these new applications arrive, it becomes increasingly important to provide an optical waveguide having a rugged physical construction. This is necessary to both simplify initial installation procedures, and insure a long service life after installation. Thus, a waveguide filament of increased mechanical strength is needed, as well as one having good resistance to environmental conditions, especially atmospheric moisture. It would be particularly desirable, from a production standpoint, to provide such service characteristics within the beneficial framework of a continuous filament drawing and processing procedure.
Presently available optical waveguide filaments are composed of a glass core, e.g., a doped fused silica, and a glass cladding, e.g., a fused silica or glass of similar characteristics. Such known composites have been processed to provide exceptionally good optical characteristics in the core member, thus providing a sound technical basis for use as a communications medium.
It is recognized, however, that, despite its great theoretical strength, glass is still a relatively fragile material in practice due to surface microcracking. Further, numerous studies have shown that moisture can be adsorbed on a glass surface and react with the silica in such surface by hydration. This leads to development of localized stress in the surface and further opening of the microcracks. Obviously, this is a continuing cycle which ultimately seriously diminishes the effectiveness of a silicate glass coating or cladding.
A glass-ceramic material is the polycrystalline product of uniform, internal, in situ crystallization of a glass by heat treatment. It has been shown that these materials are frequently stronger mechanically than their parent glasses. Studies have also shown that at least some glass-ceramics are less prone to water attack than their parent glasses.
It would then be desirable to utilize these advantageous features of glass-ceramics in the encasing of optical waveguides. However, certain rather basic considerations have made this seem quite impractical.
A major concern is that optical filaments are customarily produced by forming a blank or preform of the composite in the shape of a rod, or cylinder, heating at least a zone of such blank to drawing temperature, and then drawing in usual filament forming manner. However, inasmuch as a glass-ceramic is characterized by a rather rigid crystalline structure, such drawing procedure is inapplicable to a glass-ceramic material.
It would, of course, be possible to produce the encasing glass-ceramic from a glass that could be thermally crystallized to a glass-ceramic state. However, glasses frequently undergo rather large changes in coefficient of thermal expansion during conversion to the glass-ceramic state. Thus, while the ultimate expansion match, or controlled mismatch, in the glass-ceramic state may be satisfactory, it may be quite impractical to produce a seal between the glasses. For these and other reasons then, a glass-ceramic protective and strengthening outer coating has not been considered practical for a waveguide.